The present invention relates to the field of paste making processes for lead-acid batteries.
Battery plates are conventionally produced using a process that requires several heating, cooling, and handling steps. Under a generally followed batch process, a minimum batch size of about 2,400 to about 3,000 lbs. of dry leady oxide is typically used to produce about 7,000 to about 10,000 battery plates. The leady oxide is originally in powder form, and mixed with water to form aqueous slurry, and then reacted with a strong acid to produce a paste. The paste material is pressed onto an expanded grid, cut to a particular dimension, and flash dried at a high temperature. The plates thus produced are stacked on skids and transported to a steam chamber (for positive plates only) and eventually to a large curing room, where the skids of plates are cured for at least 3 to 4 days. Each skid holds between about 5,000 and about 10,000 plates. Once cured, the plates are retrieved and transported to a green group assembly.
Due to the long production time and stacking arrangement of the above process, it is commonly found that the quality, as well as the physical characteristics of the cured battery plates, varies widely. For example, the plates produced at the beginning of the batch are often of a different quality or are different physically from plates that are produced at the end of a batch. Furthermore, the plates often are found to vary considerably within a single plate stack.
Batches of the green batteries thus produced are sent to a formation area for material activation. Formation time is typically between about 18 and about 30 hours. The final process for finishing the formed batteries involves the steps of dumping all forming acid, refilling the batteries with the shipping acid, and sealing the batteries with a final cover.
The typical production time for a single battery, from mixing the oxide powders to finishing the product, is between about six and seven days. However, a production time of three to four weeks is not uncommon given the batch-and-queue operations commonly involved. Analysis of the above battery production process reveals that major bottlenecks occur at the curing step, and at the formation step.
Certain steps have been taken to provide more efficiency at the curing step. For example, curing rooms that are typically very large have been replaced with a number of small, environmental controlled curing chambers. As a result of this modification, the curing time has been significantly reduced. However, due to the cost associated with the sophisticated curing equipment, the minimum batch size at the curing step is limited to the number of plates on a skid instead of the size of a single plate stack, or even a single plate. Thus, the batch-and-queue mode must involve smaller batches, creating another efficiency limitation in the process.
There is therefore a need for an improved battery production process that involves a single piece flow instead of a flow by the batch. Accordingly, there is a need for a process that removes the batch-and-queue operation upstream in order to provide a meaningful, leaner process downstream. Any meaningful improvement in the process must involve faster initiation of reactions, and removal of excess water in a battery, and also the efficiency of these actions.
One publication discloses a continuous curing or pasting process. Pavlov, D. and P. Eirich, xe2x80x9cA New Technology for Preparation of Pastes for Lead-Acid Batteries,xe2x80x9d The Battery Man, Apr., 16 (1998), disclose a batch-type paste-making system that is based on the concept of producing basic lead sulfate complex crystals in a continuous process. The process disclosed in the publication uses a paste mixer that is a completely closed system, within which a vacuum can be created, thereby eliminating the effect of the surrounding medium on the paste particles produced by the process. The paste is produced by the addition of water to a leady oxide powder, and the subsequent reaction of H2SO4 with the leady oxide. The reaction is exothermic, and the heat released by the reaction is carried away by the endothermic process of water evaporation. Thus, the maintenance of a constant temperature in the closed system is an object, and a realization, of the process. When the temperature is maintained above 90xc2x0 C. and a certain ratio of leady oxide and sulfuric acid is provided, the entire amount of leady oxide is converted into tetrabasic lead sulfate. A vacuum created in the closed system removes moisture until the paste is of the desired density. While the closed paste formation system is a departure from conventional paste formation processes, the pasting and curing steps of the process are extremely lengthy, requiring sixteen to twenty hours for formation of the battery plates once the paste is created. Accordingly, a bottleneck occurs at the pasting/curing step of the process disclosed in the publication.
There is therefore a need, in addition to those needs set forth above, for a continuous paste making process for a lead-acid battery that significantly shortens the plate production step. Such a process must overcome the rate-limiting step in the Pavlov et al. publication of pasting and curing the plates once the battery paste is formed.
It is an object of the present invention to meet the above-described needs and others. Specifically, the continuous paste making process for lead-acid batteries that includes the steps of mixing water with a lead oxide, reacting an acid with the lead oxide in a mixture to produce lead oxide-lead sulfate compounds, and forming a paste comprising interlocking basic lead sulfate complex crystals from the lead oxide-lead sulfate compounds, in accordance with the present invention is characterized in that the mixing, reacting, and crystal forming steps occur in an extrusion or a high-shear continuous processing apparatus. The method, in accordance with the present invention, is also characterized in that it further includes the step of extruding the paste from the extrusion apparatus into a grid mesh where the paste is dried to form a battery plate of the lead-acid battery.
It is preferred that the extrusion apparatus includes a first cylindrical bore having a first helical channel disposed therein, the rotation of which causes any material in the first helical channel to be moved toward a die orifice. The extrusion apparatus also preferably includes a second cylindrical bore having a second helical channel disposed therein, the rotation of which causes any material in the second helical channel to be forced through the die orifice as part of the extruding step. At least the mixing step preferably occurs in the first cylindrical bore. The reacting step most preferably also occurs in the first cylindrical bore. According to the preferred embodiment, the first cylindrical bore must be attached to the second cylindrical bore to allow the process to be continuous, and the first helical channel is disposed upstream relative to the second helical channel. The first helical channel is preferably physically separated from the second helical channel.
The acid used in the reacting step is preferably sulfuric acid. Depending on whether a positive or negative electrode is being produced, the interlocking basic lead sulfate complex crystals are either tetrabasic lead sulfate or tribasic lead sulfate.
The method preferably further includes the step of preheating the grid mesh to approximate a temperature of the paste when the paste is extruded from the extrusion apparatus. The amount of paste, and various properties of the paste can be controlled by bringing the grid mesh into contact with the paste by placing the grid on a conveyor that moves toward the extruder die orifice. The method then preferable includes the step of adjusting a quantity of paste to be extruded into the grid mesh by adjusting a speed of the conveyor. The extruding step can be performed as a sheathing process, a roll-forming process, a tape-casting process, or an injection molding process.
The paste forming step of the present invention includes removing a portion of the water from the mixture. If necessary, the paste forming step also includes adding an additional amount of water to the mixture to provide optimal rheological properties to the paste.
The reacting step of the present invention preferably includes the introducing the acid at a plurality of locations along the length of the extrusion apparatus. Water or other desired liquid can also be introduced at various locations of the extrusion apparatus. In either case, unidirectional valves may be incorporated to introduce the acid, water or other liquid.
The method also preferably includes the step of controlling a temperature inside the extruder to optimize the reacting step. Various conventional temperature control methods can be incorporated to meet this step.
If necessary, the method of the present invention further includes the step of adding chemical binders or reactants to the mixture prior to extrusion of said paste. The binders can be included to optimize the quality of the paste for formation within the grid mesh.
Additional objects, advantages and novel features of the invention will be set forth in the description which follows or may be learned by those skilled in the art through reading these materials or practicing the invention. The objects and advantages of the invention may be achieved through the means recited in the attached claims.